I' afraid I come from a position of ignorance on bash on a mac and the commands available by default, but if you enter

locate GetOpt/Mixed.pm
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What directory is it in, if not one listed above in @INC, something went awry, try adding the following at the start of your Perl script

BEGIN{
push ( @INC, "/the/directory/I/just/found" ) ;
}
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WARNING: if there is already a BEGIN block add the above command to the end of it rather than writing a second one.